Tons of great stuff here. I'll give you my $0.02
Do not fight the fall.
Fighting to stay upright is when many injuries occur. Especially so for the beginner. Yes your bindings are supposed to release and save you, but do not depend on them. When you know you are not going to stay upright it is often better just to relax the body and lower your bottom to one side OR the other, whichever seems closer to the Earth (usually the uphill side.) DO NOT sit directly back on the tails. As with all aspects of skiing it is better to be in control and a controlled fall is always a safer fall. The more controlled the fall the less danger it presents and the quicker/easier it is to recover and get back up and skiing again.
It's when you fight the fall that bad things happen. Far too often you see a beginner, half fallen, skis pointing in opposing directions, holding themselves up by their arms, and fighting to untwist the skis. This is not a good situation. The combination of body weight and edged skis that are locked into the snow can place tremendous forces on the lower joints. Trying to stand up in order to release those edges is only going to put more force on them, further threatening those joints.
In that situation the immediate goal is to get the downforce off the skis and the best way to do that is get your upper body down on the snow, laying your torso flat if possible. Once you have released the skis/your legs from the weight of your upper body then you will find it all much easier to manipulate them and go about the business of untangling the skis.
I have long thought that we do beginners a great disservice by not spending more time teaching them how to fall and how to recover even though it is an inevitable part of the learning process. People tell me its' because they think it's discouraging but I disagree. I've been skiing almost thirty years and I still fall, because even now I'm still learning and still trying.